Two species of mangrove gastropod, Neritina (Dostia) violacea Gmelin, 1791 and N. (D.) cornucopia Benson, 1836, from Hong Kong have been compared morphologically, allozymically, and karyotypically. The two can be identified by differences in shell pigmentation patterns, ventral shell characters, and radular teeth characters. Allozymic variations at 15 presumptive gene loci were examined. Average heterozygosities were 0.058 in local populations of N. (D.) violacea and 0.138 in N. (D.) cornucopia. Six enzyme encoding loci were typically diagnostic for individuals of each species and no hybrid patterns have been recorded. Nei's standard genetic distance and genetic identity between the two species were estimated to be 0.814 and 0.443, respectively. Karyotypic investigations have revealed that both species are diplonts, with two X-chromosomes in females and one in males. Diploid numbers are 2n = 20 + XX (or XO) for N. (D.) violacea and 2n = 26 + XX (or XO) for N. (D.) cornucopia.